JP5454256B2 - Optical waveguide device and optical receiver comprising such an optical waveguide device - Google Patents

Description

  The present invention relates to an optical waveguide device and an optical receiver including such an optical waveguide device.

  In recent years, in order to increase transmission capacity in an optical communication system, an improvement in bit rate has been demanded. In order to improve the transmission capacity without increasing the bit rate, for example, multilevel phase shift keying may be used.

  Specific examples of the multilevel phase shift keying include quaternary phase shift keying (QPSK) or differential quadrature phase shift keying (DQPSK).

  In order to demodulate QPSK or DQPSK signal light, for example, a coherent optical receiver provided with an optical hybrid circuit is used. The optical hybrid circuit outputs four signal lights in accordance with the phase modulation state of the input QPSK or DQPSK signal light, and takes out the multilevel information, and is a main circuit in the coherent optical receiver. It is.

  In order to manufacture a coherent optical receiver having excellent cost performance, it is required to reduce the size of the optical hybrid circuit.

  FIG. 1 is a diagram illustrating a first example of a conventional optical hybrid circuit.

  The optical hybrid circuit 111 shown in FIG. 1 is formed by four 3 dB couplers and a 90 ° phase shifter. The 3 dB couplers or the 3 dB coupler and the 90 ° phase shifter are connected by an optical waveguide. The optical hybrid circuit 111 receives QPSK signal light and local oscillation light (LO light) using two input channels. Then, four output lights whose phases are different by 90 degrees are output from the output channels. The output light includes S-L and S + L signal lights that are in-phase components, and S-jL and S + jL signal lights that are quadrature components.

  However, since the optical hybrid circuit 111 shown in FIG. 1 has many elements forming the circuit, there is a limit to downsizing the optical hybrid circuit.

  FIG. 2 is a diagram illustrating Example 2 of an optical hybrid circuit according to the related art.

  The optical hybrid circuit 112 shown in FIG. 2 is formed by four input channels and four output channels, and a rectangular 4: 4 multimode interference (MMI) coupler. The optical hybrid circuit 112 receives QPSK signal light and LO light using two input channels that are asymmetric about the central axis in the light propagation direction among the four input channels. Then, the input signal light is self-imaged by multimode interference in the MMI coupler, and four output lights having phases different by 90 degrees are output from the output channels.

The optical hybrid circuit 112 has a simple structure as compared with the optical hybrid circuit shown in FIG. 1, and can reduce the dimension in the light propagation direction (hereinafter also simply referred to as an element length). In the rectangular optical hybrid circuit shown in FIG. 2, the element length L _MMI is proportional to the square of the width of the optical hybrid circuit (dimension in the direction orthogonal to the light propagation direction). Therefore, the rectangular optical hybrid circuit shown in FIG. 2 needs to reduce the width W _{MMI in} order to shorten the element length.

However, in order to reduce the width W _MMI while maintaining the width of the input channel, the width g between the input channels must be reduced. However, there is a limit to reducing the width g from the viewpoint of processing accuracy in a manufacturing process such as etching. Therefore, there is a limit to shortening the element length of the rectangular optical hybrid circuit shown in FIG.

  FIG. 3 is a diagram illustrating an example 3 of a conventional optical hybrid circuit.

In the optical hybrid circuit 113 shown in FIG. 3, both sides of the MMI coupler are formed in a butterfly taper shape. The width of the MMI coupler decreases in a taper shape toward the light propagation direction and then increases in a taper shape. The width on the input side of the MMI coupler is W _MMI and is the same as that of the optical hybrid circuit 112 shown in FIG. 2, but the center width in the light propagation direction is W _MB and is larger than the width W _MMI on the input side. It is narrowing. Both side portions of the MMI coupler have discontinuities at the width W _MB of the central portion. In the optical hybrid circuit 113 having such a shape, the average width is reduced to reduce the element length.

  FIG. 4 is a diagram illustrating an example 4 of a conventional optical hybrid circuit.

In the optical hybrid circuit 114 shown in FIG. 4, both side portions of the MMI coupler are formed in a parabolic shape protruding inward. The width of the MMI coupler decreases continuously in the light propagation direction and then increases continuously. The width on the input side of the MMI coupler is W _MMI and is the same as that of the optical hybrid circuit 112 shown in FIG. 2, but the center width in the light propagation direction is W _MP and is larger than the width W _MMI on the input side. It is narrowing. Both sides of the MMI coupler are also continuous in the central width W _MP portion. The optical hybrid circuit 114 having such a shape also reduces the average width to reduce the element length.

  The optical hybrid circuit shown in FIGS. 3 and 4 reduces the element length of the MMI coupler. However, for the optical hybrid circuit shown in FIGS. 3 and 4, the multilevel phase shift keying signal to which the input light is equally distributed and output, or the phase information of each output signal is input, is included. Further improvements in optical performance, such as maintaining phase information, are desired.

Special table 2001-514766

D. Hoffmann et al., “Integrated Optics Eight-Port 90 ° Hybrid on LiNbO3”, Journal of Lightwave Technology, May 1989, Vol. 7, no. 5, pp. 794-798 E. C. M.M. Pennings et al., “Ultracompact, All-Passive Optical 90 ° Hybrid on InP Using Self-Imaging”, IEEE Photonics Technology Letters, June 1993, Vol. 5, no. 6, pp. 701-703 Pierre A Besse et al., “New 2 × 2 and 1 × 3 Multimode Interface Couplers with Free Selection of Power Splitting Ratios”, Journal of Lightweight 96, V. 14, no. 10, pp. 2286-2293 D. S. Levy et al., “Length Reduction of Tapered N × N MMI Devices”, IEEE Photonics Technology Letters, June 1998, Vol. 10, no. 6, pp. 830-832

  An object of the present specification is to provide an optical waveguide device having a small size and excellent optical performance.

  In order to solve the above problems, according to one mode of the optical waveguide element disclosed in the present specification, a plurality of input channels, a plurality of output channels, a plurality of input channels are connected to one end, and the other A multimode interference coupler having a plurality of output channels connected to an end of the multimode interference coupler, the multimode interference coupler comprising: a first portion whose width gradually decreases from one end toward the other end; A second part extending from one end side toward the other end side while maintaining the width of the connecting part connected to the one part, and connected to the second part from the one end side to the other end part A third portion whose width gradually increases.

  According to one form of the optical waveguide element disclosed in the present specification described above, the optical waveguide element has a small size and excellent optical performance.

  The objects and advantages of the invention will be realized and obtained by means of the elements and combinations particularly pointed out in the appended claims.

  Both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention as claimed.

It is a figure which shows Example 1 of the optical hybrid circuit by a prior art. It is a figure which shows Example 2 of the optical hybrid circuit by a prior art. It is a figure which shows Example 3 of the optical hybrid circuit by a prior art. It is a figure which shows Example 4 of the optical hybrid circuit by a prior art. 1 is a diagram illustrating a first embodiment of an optical hybrid circuit disclosed in this specification. FIG. It is a figure explaining the wave front which propagates the inside of the optical hybrid circuit shown in FIG. (A)-(E) is a figure which shows the relationship between the transmittance | permeability of each output channel at the time of changing the length of the 2nd part of the multimode interference coupler of an optical hybrid circuit, and a wavelength. (A)-(E) is a figure which shows the relationship between the phase shift and wavelength of each output channel at the time of changing the length of the 2nd part of the multimode interference coupler of an optical hybrid circuit. It is a figure explaining the wave front which propagates the inside of the optical hybrid circuit shown in FIG. (A) is a figure which shows the relationship between the transmittance | permeability and wavelength of each output channel of the optical hybrid circuit shown in FIG. 4, (B) is a figure which shows the relationship between the phase shift of each output channel, and a wavelength. It is. It is a figure explaining the wave front which propagates the inside of the optical hybrid circuit shown in FIG. FIG. 5 is a diagram comparing the shortening rate of the optical hybrid circuit of the first embodiment with that of the optical hybrid circuit shown in FIGS. 3 and 4. It is a figure which shows the relationship between the shortening rate of the optical hybrid circuit of 1st Embodiment, and the length of a 2nd part. It is a figure which shows the relationship between the shift | offset | difference of the phase of each output channel when a shortening rate of the optical hybrid circuit of 1st Embodiment is 0.58, and a wavelength. FIG. 5 is a diagram showing the relationship between the phase shift of each output channel and the wavelength when the shortening rate of the optical hybrid circuit shown in FIG. 4 is 0.58. FIG. 6 is an enlarged sectional view taken along line XX in FIG. 5. It is a figure which shows 2nd Embodiment of the optical hybrid circuit disclosed by this specification. (A) is a figure which shows the relationship of the transmittance | permeability and wavelength of each output channel of the optical hybrid circuit of 1st Embodiment, (B) is transmission of each output channel of the optical hybrid circuit of 2nd Embodiment. It is a figure which shows the relationship between a rate and a wavelength. (A) is a figure which shows the relationship between the phase shift and wavelength of each output channel of the optical hybrid circuit of 1st Embodiment, (B) is the phase shift of each output channel of 2nd Embodiment, and It is a figure which shows the relationship with a wavelength. It is a figure which shows 3rd Embodiment of the optical hybrid circuit disclosed by this specification. (A) is a figure which shows the relationship of the transmittance | permeability and wavelength of each output channel of the optical hybrid circuit of 1st Embodiment, (B) is transmission of each output channel of the optical hybrid circuit of 3rd Embodiment. It is a figure which shows the relationship between a rate and a wavelength. (A) is a figure which shows the relationship between the phase shift and wavelength of each output channel of the optical hybrid circuit of 1st Embodiment, (B) is the phase shift of each output channel of 3rd Embodiment, and It is a figure which shows the relationship with a wavelength. It is a figure which shows one Embodiment of the optical receiver disclosed to this specification. It is a figure which shows the transmittance | permeability in each output channel when QPSK signal light is input into the optical receiver shown in FIG. It is a figure which shows other embodiment of the optical receiver disclosed by this specification.

  An optical hybrid circuit as an optical waveguide device disclosed in the present specification inputs multilevel phase shift keyed signal light and demodulates a multilevel signal by changing the phase from the input signal. Preferably used. The optical hybrid circuit disclosed in this specification is used to demodulate, for example, multilevel phase shift keyed signal light such as BPSK, QPSK, and 8PSK, or multilevel amplitude phase modulated signal light such as 16QAM and 64QAM. it can. In the following description, an optical hybrid circuit in the case of demodulating QPSK signal light will be described as an example. The number of input channels and output channels included in the optical hybrid circuit can be appropriately set according to the input signal light.

  Hereinafter, a preferred first embodiment of an optical hybrid circuit as an optical waveguide device disclosed in the present specification will be described with reference to the drawings. However, it should be noted that the technical scope of the present invention is not limited to these embodiments, but extends to the invention described in the claims and equivalents thereof.

  FIG. 5 is a diagram illustrating a first embodiment of the optical hybrid circuit disclosed in this specification.

  The optical hybrid circuit 10 of this embodiment includes four input channels 11, four output channels 12, four input channels 11 connected to one end 14, and four output channels 12 to the other end 15. Is connected to the multimode interference coupler 13. The multimode interference coupler 13 propagates light from one end 14 toward the other end 15.

  The multimode interference coupler 13 includes a first portion 13a whose width gradually decreases from one end portion 14 toward the other end portion 15 side, and one of the multi-mode interference couplers 13 connected to the first portion 13a while maintaining the width of the connection portion. A second portion 13b extending from the end portion 14 side toward the other end portion 15 side, and a third portion connected to the second portion 13b and gradually increasing in width from the one end portion 14 side toward the other end portion 15 And a portion 13c. The width of the connection portion between the first portion 13a and the second portion 13b is the same as the width of the connection portion between the second portion 13b and the third portion 13c. In this specification, the direction from one end 14 to the other end 15 in the multimode interference coupler 13 is also referred to as a light propagation direction.

  The width of the first portion 13a of the multimode interference coupler 13 is defined by a pair of opposing side portions 13e symmetrical to the central axis CL in the width direction, and the shape of each of the pair of side portions 13e is a straight line. Here, the width of the first portion 13a is a length in a direction orthogonal to the light propagation direction in the first portion 13a. This also applies to the width of the second portion 13b and the width of the third portion 13c.

One open end 14 of the first portion 13a has a width _WS , and the four input channels 11 are connected to the end 14. The four input channels 11 are arranged symmetrically and at equal intervals with respect to the center axis CL in the width direction of the optical hybrid circuit 10. The length of the first portion 13a in the light propagation direction is _LM1 .

  The width of the third portion 13c of the multimode interference coupler 13 is defined by a pair of opposing side portions 13f that are symmetrical to the central axis CL in the width direction, and the shape of each of the pair of side portions 13f is also a straight line.

The other open end 15 of the third portion 13 c also has a width _WS , and the four output channels 12 are connected to this end 15. The four output channels 12 are arranged symmetrically and at equal intervals with respect to the center axis CL in the width direction of the optical hybrid circuit 10. In FIG. 5, the four output channels are numbered Ch-1, Ch-2, Ch-3, and Ch-4.

The second portion 13b sandwiched between the first portion 13a and the third portion 13c has a rectangular shape. The width of the second portion 13b is _{W M,} the length of the light propagation direction of the second portion 13b is _{L ST.}

The width of the first portion 13a is toward the light propagation direction, it decreases in a tapered shape from _{W S} to _{W M.} The width of the third portion 13c increases in a tapered shape from W _M to W _{S in} the light propagation direction.

  In the optical hybrid circuit 10, the first portion 13a and the third portion 13c have the same length in the light propagation direction.

  In the optical hybrid circuit 10, the first portion 13 a and the third portion 13 c are formed symmetrically with respect to the central axis (not shown) of the optical hybrid circuit 10 in the light propagation direction. Therefore, the four input channels 11 and the four output channels 12 are also formed symmetrically with respect to the central axis (not shown) in the light propagation direction.

  In FIG. 5, the direction from one end portion 14 to the other end portion 15 in the multimode interference coupler 13 is represented by a positive z-axis direction.

The optical hybrid circuit 10 shown in FIG. 5, the length of the z-axis direction of the combined first portion 13a and second portion 13b is _{L M2.} Further, the length of the multi-mode interference coupler 13 in the z-axis direction is represented by _LM3 .

  QPSK signal light and LO light are input to the optical hybrid circuit 10 using two input channels asymmetric with respect to the central axis CL in the width direction among the four input channels 11. For example, the other two input channels that are not used may not be formed. In this case, the optical hybrid circuit 10 includes two input channels 11 and four output channels 12.

  As described above, the optical hybrid circuit 10 receives the QPSK signal light and the LO light using two input channels that are asymmetric with respect to the central axis in the optical propagation direction. The QPSK signal light and LO light input from the input channel 11 are self-imaged by multimode interference based on general mode interference in the multimode interference coupler 13, and four different signal lights are output from the output channels 12, respectively. The

  It is preferable that the optical hybrid circuit 10 has an optical performance of outputting light input from any one of the input channels by equally branching from each output channel. In addition, the optical hybrid circuit 10 preferably has an optical performance with a small phase shift between the phase of each signal light output from the output channel and the phase of the input QPSK signal light.

The optical hybrid circuit 10, by adjusting the length L _ST of the light propagation direction of the second portion, the length of the light propagation direction of the multimode interference coupler 13 (hereinafter, simply referred to as element length) as well as shorten the, Provides excellent optical performance.

Specifically, the length L _ST of the second portion 13b of the multimode interference coupler 13 in the light propagation direction is preferably determined as follows. The QPSK signal light is input to any one of the four input channels 11, and the difference in light intensity between the signal lights output from the four output channels 12 is based on the light intensity of the input QPSK signal light. The length _LST is determined to be within 3 dB. More preferably, the length L _ST is determined such that the difference in light intensity between the signal lights output from the four output channels 12 is within 2 dB with reference to the light intensity of the input QPSK signal light. More preferably, the length L _ST is determined such that the difference in light intensity between the signal lights output from the four output channels 12 is within 1 dB with reference to the light intensity of the input QPSK signal light.

It is also preferable that the length L _ST of the second portion 13b of the multimode interference coupler 13 in the light propagation direction is determined as follows. The length _LST is determined so that the phase shift of the signal light output from the four output channels is within ± 10 degrees. Specifically, if the output signal light is an in-phase component, the length L _ST is preferably determined so that the phase of the signal light is within ± 10 degrees with respect to 0 or 180 degrees. If the output signal light is a quadrature component, it is preferable that the length L _ST is determined so that the phase of the signal light is within ± 10 degrees with respect to 90 or 270 degrees. More preferably, the length L _ST is determined so that the phase shift of the signal light output from the four output channels is within ± 5 degrees. Specifically, if the output signal light is an in-phase component, the length L _ST is preferably determined so that the phase of the signal light is within ± 5 degrees with respect to 0 or 180 degrees. If the output signal light is a quadrature component, the length L _ST is preferably determined so that the phase of the signal light is within ± 5 degrees with respect to 90 or 270 degrees.

  Next, the element length of the multimode interference coupler 13 of the optical hybrid circuit 10 will be described below.

First, the relationship between the optimum element length L _{MMI of the} rectangular multimode interference coupler shown in FIG. 2 and the width W _MMI of the multimode interference coupler will be described. Next, the element length of the multimode interference coupler 13 of the optical hybrid circuit 10 is expressed using the element length L _MMI of the rectangular multimode interference coupler.

The optimum element length L _MMI of the rectangular multimode interference coupler shown in FIG. 2 is determined depending on the width W _MMI of the multimode interference coupler, the refractive index of the waveguide, the number of excitation modes, the interference mechanism, and the like. The relationship between the optimum element length L _MMI of this multimode interference coupler and the width W _MMI of the multimode interference coupler is obtained as follows.

First, in the rectangular multimode interference coupler shown in FIG. 2, the propagation constant β _ν (ν: the order of the propagation mode) in an arbitrary mode propagating in the multimode interference coupler is simplified as shown in the equation (1). expressed.

Here, k ₀ is the wave number of signal light in vacuum, N _eq is the refractive index of the waveguide in the multimode interference coupler, and λ is the wavelength of the signal light. In this case, the difference in the propagation constant between the fundamental mode excited in the multimode interference coupler and an arbitrary higher-order mode is expressed as shown in Equation (2).

Here, _Lπ is the beat length of the multimode interference coupler. In the case of the rectangular multimode interference coupler shown in FIG. 2, this L _π is approximated as shown in equation (3) using equation (2).

In this way, the relationship between the optimum element length L _{MMI of the} rectangular multimode interference coupler shown in FIG. 2 and the width W _MMI of the multimode interference coupler is obtained as shown in Equation (3a).

Next, using the device length L _MMI of the rectangular multi-mode interference coupler, determining the relationship between the length L _ST of the multi-element length mode interference coupler 13 and the second portion 13b of the optical hybrid circuit 10 shown in FIG. 5 .

First, since the width of the multimode interference coupler 13 shown in FIG. 5 is not constant in the z-axis direction, the difference in propagation constant between the fundamental mode and an arbitrary higher-order mode changes in the z-axis direction. Therefore, the difference in propagation constants between the fundamental mode and any higher-order mode, the 0 in the z-axis direction by integrating the area up to L _M3, phase variation Δρ in the multi-mode interference coupler 13 is formula (4) It is expressed as follows.

Here, W _M (z) represents the width of the multimode interference coupler 13 as a function of z.

The function W _M (z) is expressed as in Expression (5a), Expression (5b), and Expression (5c) in each of the three regions of the z axis.

Further, the length L _ST of the second portion of the multimode interference coupler 13 in the z-axis direction is expressed as in Expression (6) using the length L _M3 .

  Here, the parameter f is a real number of 0 or more.

  Substituting Equation (5a), Equation (5b), and Equation (5c) into Equation (4) for integration, and then considering the relationship of Equation (6), Equation (7) is obtained.

Here, χ ^T expressed by Expression (8) is a constant depending on the shape of the multimode interference coupler 13, and is determined by W _S and W _M representing the width of the multimode interference coupler 13 and the parameter f.

From the equations (3) and (7), the relationship between the beat lengths L _Tπ and χ ^T of the multimode interference coupler 13 of the optical hybrid circuit 10 shown in FIG. 5 is obtained as in the equation (9).

In this way, by using a beat length L _[pi of the rectangular multi-mode interference coupler may represent an optimal beat length L _Tipai of the multimode interference coupler 13 of the optical hybrid circuit 10.

As shown in Expression (9), when the beat length L _π of the rectangular multimode interference coupler is constant, the beat length L _Tπ of the multimode interference coupler 13 and χ ^T have an inversely proportional relationship. Accordingly, it can be seen that the element length L _M3 of the multimode interference coupler 13 shown in FIG. 5 decreases as χ ^T increases. That is, when the optical hybrid circuit 10, the width W _S of the first portion 13a and the third portion of the multimode interference coupler 13, and the same as the width W _MMI of the rectangular multi-mode interference coupler shown in FIG. 2, The element length L _M3 of the multimode interference coupler 13 is shortened to a ratio of 1 / χ ^T than the element length L _MMI of the multimode interference coupler shown in FIG.

This χ ^T is determined by determining the parameter f together with W _S and W _M representing the width of the multimode interference coupler 13. Similarly, the length L _ST of the second portion 13b is also determined by the parameter f as shown in Expression (6). Accordingly, if W _S and W _M are constant, the parameter f is determined by determining χ ^T, and therefore the length L _ST of the second portion 13b is also determined _. Here, the beat length L _Tπ is used as the element length L _M3 in Equation (6).

  FIG. 6 is a diagram schematically illustrating a wavefront propagating in the multimode interference coupler 13 of the optical hybrid circuit 10 shown in FIG.

  As shown in FIG. 6, in the optical hybrid circuit 10, the wavefront of the signal light incident from the input channel 11 into the multimode interference coupler 13 is bent concentrically and propagates through the multimode interference coupler 13. The wavefront propagating in the multimode interference coupler 13 is shifted in phase due to the discontinuity of the wave number vector of the propagated signal light in the central portion of the multimode interference coupler 13. In order to obtain good optical characteristics of the optical hybrid circuit 10, it is preferable that this phase shift is small.

In the optical hybrid circuit 10, this phase shift is compensated by adjusting the length L _ST of the second portion 13 b of the multimode interference coupler 13.

  Next, a calculation example of the optical characteristics of the optical hybrid circuit 10 will be described below with reference to the drawings.

FIGS. 7A to 7E show the relationship between the transmittance Tr and the wavelength λ of each output channel when the length L _ST of the second portion 13b of the multimode interference coupler 13 of the optical hybrid circuit 10 is changed. It is a figure which shows a relationship. 7A to 7E, signal light is input from one of the four input channels 11, and the transmittance Tr of the signal light output from each of the four output channels is input. The result calculated for the wavelength λ of the signal light is shown by a solid line. 7A to 7E, signal light is input from another input channel, and the signal light transmittance Tr output from each of the four output channels is input signal light. The results calculated for the wavelength λ are shown in dashed lines. That is, in FIGS. 7A to 7E, the transmittance Tr is indicated by four solid lines and four chain lines. In this transmittance Tr, the light intensity of each signal light output from the four output channels 12 is shown in units of dB with reference to the light intensity of the input QPSK signal light.

Further, FIG. 8 (A) ~ FIG 8 (E) includes a shift the wavelength of the phase of each output channel when changing the length L _ST of the second portion 13b of the multimode interference coupler of the optical hybrid circuit 10 lambda It is a figure which shows the relationship. 8A to 8E, the QPSK signal light and the LO light are input to the two input channels 11, and the phase shift Δψ calculated from the intensity of each signal light output from the four output channels. Shows the result calculated for the wavelength λ of the input signal light. Specifically, if the output signal light is an in-phase component, the difference between the phase of the signal light and 0 or 180 degrees means a phase shift Δψ. If the output signal light is a quadrature component, the difference between the phase of the signal light and 90 or 270 degrees means a phase shift Δψ. In FIGS. 8A to 8E, the operating band is indicated by a chain line where the phase deviation is ± 5 degrees.

The results shown in FIGS. 7A to 7E and FIGS. 8A to 8E were calculated using a beam propagation method (BPM). In the calculation of BPM, 3.24 was used as the equivalent refractive index of the waveguide region of the multimode interference coupler, and 1.0 was used as the refractive index of the region other than the waveguide. Further, the dimensions of each element forming the optical hybrid circuit 10 has a width of 2.0μm input channels 11 and output channels 12, input and output spacing between channels 2.3 .mu.m, the width _{W S} 17.2Myuemu, width W _M was 13.2 μm.

In the calculation of BPM, first, 1 / χ ^T is determined as the shortening rate, and then the beat length L _Tπ is determined. Next, by changing the length L _ST and the width W _M of the second portion 13b, the element length L _M3 was optimized to show the best transmittance for each L _ST . Eventually, an f value that is an arbitrary real number is determined, an appropriate element length L _M3 for the f value is obtained, and the best device in terms of transmittance and phase shift characteristics may be selected for each device.

Specifically, the element length L _{M3 in} FIGS. 7A and 8A is 477.5 μm, and the length L _ST is 0 μm. The element length L _M3 in FIGS. 7B and 8B is 462.8 μm, and the length L _ST is 50 μm. The element length L _M3π in FIGS. 7C and _8C is 449.0 μm, and the length L _ST is 100 μm. The element length L _M3 in FIGS. 7D and 8D is 446.3 μm, and the length L _ST is 110 μm. The element length L _M3 in FIGS. 7E and 8E is 436.0 μm, and the length L _ST is 150 μm.

In FIG. 7 (A) ~ FIG 7 (E) and FIG. 8 (A) ~ FIG 8 (E), the length _{L ST} of the second portion 13b is, in particular in the range of 110μm from 100 [mu] m, excellent transmittance and less A phase shift is obtained. That is, the input signal light is equally branched to each of the four output channels over a wide wavelength range, and signal light having a small phase shift is output. That is, it can be seen that if the length L _ST of the second portion 13b is in the range of 100 μm to 110 μm, the optical hybrid circuit 10 has good optical performance in the C band region.

Here, FIGS. 7A and 8A show the deviation of the transmittance and phase in the case of the optical hybrid circuit shown in FIG. 3 in which the length L _ST of the second portion 13b is 0 μm. ing. As shown in FIGS. 7A and 8A, it can be seen that the optical characteristics of the optical hybrid circuit shown in FIG. 3 are greatly reduced when the element length is shortened.

  FIG. 9 is a diagram for explaining a wavefront propagating in the optical hybrid circuit shown in FIG.

  As schematically shown in FIG. 9, in the optical hybrid circuit 113 shown in FIG. 3, the wavefront of the signal light incident from the input channel into the multimode interference coupler is curved concentrically and propagates in the multimode interference coupler. To do. Then, the wavefront propagating in the multimode interference coupler causes a phase shift at the discontinuous points on both sides of the multimode interference coupler. Therefore, the optical hybrid circuit 113 shown in FIG. 3 exhibits optical characteristics as shown in FIGS. 7A and 8A.

On the other hand, as described with reference to FIG. 6, the optical hybrid circuit 10 provides the second portion 13 b of the multimode interference coupler 13, thereby causing a phase generated between the first portion 13 a and the third portion 13 c. Compensate for deviations. Then, by increasing the length L _ST of the second portion 13b, the amount of compensation is the phase increases. In the examples of FIGS. 7A and 7B and FIGS. 8A and 8B, the length L _ST of the second portion 13b is small, so that the amount of phase to be compensated is insufficient. It is thought that. In the examples of FIGS. 7C and 7D, and FIGS. 8C and 8D, the amount of phase compensated by the length L _ST of the second portion 13b may cause the phase shift. It is considered to be in a corresponding state. On the other hand, in the example shown in FIG. 7 (E) and FIG. 8 (E), since the length L _ST of the second portion 13b is too large, it considered excessively in a state in which the phase shift is compensated.

10A is a diagram showing the relationship between the transmittance and wavelength of each output channel of the conventional optical hybrid circuit shown in FIG. 4, and FIG. 10B is the phase shift and wavelength of each output channel. It is a figure which shows the relationship. 10A and 10B, the dimensions of each element forming the optical hybrid circuit are as follows: the width of the input channel 11 and the output channel 12 is 2.0 μm, and the distance between the input and output channels is 2.3 μm. The width W _MMI was 17.2 μm and the width W _MP was 13.2 μm. The reduction rate of the element length was 70%.

  As shown in FIG. 10 (A), the optical hybrid circuit shown in FIG. 4 has good transmittance over a wide wavelength range, but as shown in FIG. 10 (B), the phase shift is ± It can be seen that the range within 5 degrees is significantly narrowed.

  FIG. 11 is a diagram for explaining the wavefront propagating in the optical hybrid circuit shown in FIG.

  As schematically shown in FIG. 11, in the optical hybrid circuit 114 shown in FIG. 4, the wavefront of the signal light incident from the input channel into the multimode interference coupler is curved concentrically and propagates through the multimode interference coupler. To do. The wavefront propagating in the multimode interference coupler exits from the output channel without causing a phase shift. However, in the optical hybrid circuit 114 shown in FIG. 4, as shown in FIG. 10B, since the optical characteristics are not good with respect to the phase shift, for example, good optical performance cannot be obtained in the C band region.

  FIG. 12 is a diagram comparing the shortening rate of the optical hybrid circuit of the first embodiment with that of the optical hybrid circuit shown in FIGS. 3 and 4.

In Figure 12, the optical hybrid circuit 10 shown in FIG. 5, the difference between the width W _S and a width W _M, the relationship between the shortening rate Re of the device length is indicated by the curve C1. The curve C1 was calculated using BPM under the same conditions as in FIGS. 7C and 8C. Here, the shortening rate Re is a 1 / χ ^T.

As shown in FIG. 12, the curve C1 decreases almost linearly. Change of the curve C1, as shown in equation (8), can be understood from the difference between the width W _S and the width W _M is 1 / chi ^T decreases with the increase.

Further, in FIG. 12, for the optical hybrid circuit shown in FIGS. 3 and 4, the difference between the width W _S and a width W _M, the relationship between the shortening rate Re of the device length is indicated by the curve C2 and C3 . Also for the optical hybrid circuits shown in FIGS. 3 and 4, the shortening rate is derived in the same manner as using Equation (9).

  From the comparison between the curve C1 and the curve C2 in FIG. 12, the optical hybrid circuit 10 shown in FIG. 5 has an excellent shortening rate with respect to the same horizontal axis value as the optical hybrid circuit shown in FIG. I understand.

Furthermore, the relationship between the element length of the fractional shortening the length L _ST of the optical hybrid circuit 10 shown in FIG. 5, FIG. 13 shows results of a comparison on the basis of the optical hybrid circuit shown in FIG.

Figure 13 is a diagram showing the relationship between the length L _ST of the device length shortening rate Re and second portions of the optical hybrid circuit of the first exemplary embodiment.

In FIG. 13, when the length _LST is zero, it corresponds to the optical hybrid circuit shown in FIG. As shown in FIG. 13, the length _LST is increased, and the element length shortening rate Re is linearly decreased. That is, the element length of the optical hybrid circuit 10 shown in FIG. 5 can be further reduced from the element length of the optical hybrid circuit shown in FIG. 3 as the length L _ST is increased.

  Also, as shown in FIG. 12, the optical hybrid circuit shown in FIG. 4 has an element length reduction rate equivalent to that of the optical hybrid circuit 10 shown in FIG. However, as shown in FIG. 10B, the optical hybrid circuit shown in FIG. 4 has poor optical characteristics with respect to phase shift when the element length is shortened.

Further, as shown in FIG. 12, the optical hybrid circuit 10 shown in FIG. 5, when the length _{L ST} of the second portion 13b in 100~110μm _is W S _-W M = fractional shortening when 4 [mu] m Re Is about 0.70, and the element length is reduced by about 30% compared to the rectangular multimode interference coupler. Then, as shown in FIG. 12, by increasing further the difference between the width W _S and a width W _M, it is possible to further reduce the shortening rate.

  However, the element length reduction ratio Re of the multimode interference coupler and the optical characteristics such as the operation bandwidth of the optical hybrid circuit 10 are generally in a trade-off relationship. An example in which the shortening rate of the optical hybrid circuit 10 is further reduced will be described below with reference to the drawings.

FIG. 14 is a diagram showing the relationship between the phase shift of each output channel and the wavelength when the shortening rate Re of the optical hybrid circuit 10 is 0.58. The optical characteristics shown in FIG. 14 were calculated for the case of LST = 80 μm and W _S −W _M = 6 μm.

  As shown in FIG. 14, the operating bandwidth of the optical hybrid circuit 10 is smaller than the operating bandwidth shown in FIGS. 8C and 8D.

  FIG. 15 is a diagram showing the relationship between the phase shift of each output channel and the wavelength when the shortening rate of the optical hybrid circuit shown in FIG. 4 is 0.58.

  As shown in FIG. 15, the optical hybrid circuit shown in FIG. 4 has a larger phase shift than the optical hybrid circuit 10 shown in FIG. 5 when the shortening rate is 0.58. Therefore, the optical hybrid circuit 10 of this embodiment has a wider operating bandwidth than the optical hybrid circuit shown in FIG. 4 although the operating bandwidth decreases when the shortening rate is further reduced from 0.70. I understand that

  16 is an enlarged sectional view taken along line XX in FIG.

  In the optical hybrid circuit 10, a lower clad layer 41 is laminated on a substrate 40, a core layer 42 is laminated on the lower clad layer 41, and an upper clad layer 43 is formed on the core layer 42. A mesa portion 44 is formed by the lower cladding layer 41, the core layer 42, and the upper cladding layer 43. In the optical hybrid circuit 10, the lower cladding layer 41 and the substrate 40 are integrally formed.

  The sectional view shown in FIG. 16 is a sectional view of the second portion 13b of the multimode interference coupler 13, but the first portion 13a, the third portion 13c, the input channel 11 and the output channel 12 also have the same sectional structure. That is, the thicknesses of the lower cladding layer 41, the core layer 42, and the upper cladding layer 43 are constant throughout the optical hybrid circuit 10.

  The optical hybrid circuit 10 shown in FIG. 5 is formed as follows, for example.

  First, the core layer 41 is laminated on the substrate 40 by, for example, metal organic vapor phase epitaxy (hereinafter also referred to as MOVPE). As the substrate 40, an n-type InP substrate or an undoped InP substrate can be used. As a material for forming the core layer 42, undoped GaInAsP (emission wavelength: 1.30 μm) can be used. The thickness of the core layer 42 can be 0.3 μm, for example.

  Next, the upper cladding layer 43 is epitaxially stacked on the core layer 42. As a material for forming the upper cladding layer 43, undoped or p-type InP can be used. The thickness of the upper cladding layer 43 can be set to 2.0 μm, for example.

  Next, a mask layer made of an SiO 2 film or the like is formed on the upper cladding layer 43.

  Next, the formation region of the optical hybrid circuit in the mask layer is patterned by using an optical exposure process.

  Next, using the mask layer as a mask, the upper cladding layer 43, the core layer 42, and the substrate 40 are etched to form a mesa portion 44. As shown in FIG. 16, the substrate 40 is etched from the surface of the substrate 40 to an intermediate depth to form a convex lower cladding layer 41. As an etching method, for example, dry etching such as ICP reactive ion etching can be used. In addition, the height of the mesa unit 44 can be set to, for example, 3.0 μm.

  Then, the mask layer on the upper cladding layer 43 is removed, and the optical hybrid circuit 10 is formed.

  In the method for forming the optical hybrid circuit 10 described above, an example in which InP of a III-V group compound semiconductor is used as a forming material is shown. However, the forming material is not limited to these materials, and the III-V group compound The optical hybrid circuit may be formed using semiconductor GaAs, group IV semiconductor Si, or the like.

  According to the above-described optical hybrid circuit 10 of the present embodiment, the optical hybrid circuit 10 has small dimensions and excellent optical performance.

  Moreover, the optical hybrid circuit 10 of this embodiment is suitable for monolithic integration. As described above, the element length of the multimode interference coupler 13 of the optical hybrid circuit 10 can be shortened by at least about 30% while maintaining good optical characteristics.

  In addition, the optical hybrid circuit 10 can reduce the element length of the multimode interference coupler without reducing the interval between the input channel and the output channel. Therefore, it is possible to form the optical hybrid circuit 10 using a manufacturing process having a conventional processing accuracy.

  Next, optical hybrid circuits as optical waveguides according to the second and third embodiments disclosed in the present specification will be described below with reference to the drawings. Regarding points that are not particularly described in the second and third embodiments, the description in detail regarding the first embodiment is applied as appropriate. 17 and 20, the same components as those in FIG. 5 are denoted by the same reference numerals.

  FIG. 17 is a diagram illustrating a second embodiment of the optical hybrid circuit disclosed in this specification.

  In the optical hybrid circuit 100 of the present embodiment, the shape of each of the pair of side portions 13e that defines the width of the first portion 13a of the multimode interference coupler 13 is a parabola that protrudes inward. Similarly, in the optical hybrid circuit 10, the shape of each of the pair of side portions 13f that define the width of the third portion 13c of the multimode interference coupler 13 is also a parabola convex inward.

  Other parts of the optical hybrid circuit 100 are the same as those in the first embodiment described above.

  Next, the optical characteristics of the optical hybrid circuit 100 of this embodiment will be compared with the first embodiment described above with reference to the drawings.

  FIG. 18A is a diagram showing the relationship between the transmittance and wavelength of each output channel of the optical hybrid circuit of the first embodiment, and FIG. 18B is a diagram of the optical hybrid circuit 100 of the second embodiment. It is a figure which shows the relationship between the transmittance | permeability of each output channel, and a wavelength.

  FIG. 18A was calculated under the same conditions as FIG. FIG. 18B was calculated under the same conditions as FIG. 18A except that the shapes of the side portions of the first portion 13a and the third portion 13c were different.

  The transmittance of the first embodiment is superior to the transmittance of the optical hybrid circuit 100 of the present embodiment. That is, in the optical hybrid circuit of the first embodiment, the input signal light is equally branched to each output channel.

  FIG. 19A is a diagram showing the relationship between the phase shift of each output channel and the wavelength of the optical hybrid circuit of the first embodiment, and FIG. 19B is the optical hybrid circuit 100 of the second embodiment. It is a figure which shows the relationship between the phase shift of each output channel, and a wavelength.

  FIG. 19 (A) was calculated under the same conditions as FIG. 8 (C). FIG. 19B was calculated under the same conditions as FIG. 19A except that the shapes of the side portions of the first portion 13a and the third portion 13c were different.

  The first embodiment has a smaller phase shift of each output signal light than the optical hybrid circuit 100 of the present embodiment. The output signal light of the optical hybrid circuit of the first embodiment maintains the phase of the input signal light with higher accuracy.

  FIG. 20 is a diagram illustrating a third embodiment of the optical hybrid circuit disclosed in this specification.

In the optical hybrid circuit 200 of the present embodiment, the length L _M1 of the first portion 13a of the multimode interference coupler 13 in the light propagation direction is longer than the length of the third portion 13c in the light propagation direction (L _M3 -L _M2 ). Also short. That is, in the optical hybrid circuit 200, the first portion 13a and the third portion 13c are formed asymmetrically with respect to the central axis (not shown) of the optical hybrid circuit 200 in the light propagation direction.

  Each output channel 12 is connected in the width direction of the other end 15 at the same rate as the interval in which each input channel 11 is connected in the width direction of one end 14.

  Other parts of the optical hybrid circuit 100 are the same as those in the first embodiment described above.

  Next, the optical characteristics of the optical hybrid circuit 200 of this embodiment will be compared with the first embodiment described above with reference to the drawings.

  FIG. 21A is a diagram showing the relationship between the transmittance and wavelength of each output channel of the optical hybrid circuit of the first embodiment, and FIG. 21B is a diagram of the optical hybrid circuit 200 of the third embodiment. It is a figure which shows the relationship between the transmittance | permeability of each output channel, and a wavelength.

  FIG. 21A was calculated under the same conditions as FIG. FIG. 21B was calculated under the same conditions as FIG. 21A except that the lengths of the first portion 13a and the third portion 13c in the light propagation direction are different.

  The transmittance of the first embodiment is slightly better than the transmittance of the optical hybrid circuit 200 of the present embodiment. That is, in the optical hybrid circuit of the first embodiment, the input signal light is equally branched to each output channel.

  FIG. 22A is a diagram showing the relationship between the phase shift of each output channel and the wavelength of the optical hybrid circuit of the first embodiment, and FIG. 22B is the optical hybrid circuit 200 of the third embodiment. It is a figure which shows the relationship between the phase shift of each output channel, and a wavelength.

  FIG. 22 (A) was calculated under the same conditions as FIG. 8 (C). FIG. 22B was calculated under the same conditions as FIG. 22A except that the lengths of the first portion 13a and the third portion 13c in the light propagation direction are different.

  The first embodiment has a smaller phase shift of each output signal light than the optical hybrid circuit 200 of the present embodiment. The output signal light of the optical hybrid circuit of the first embodiment maintains the phase of the input signal light with higher accuracy.

  Next, an optical receiver including the above-described optical hybrid circuit disclosed in this specification will be described below with reference to the drawings. FIG. 23 is a diagram illustrating an embodiment of a coherent optical receiver disclosed in the present specification.

  The coherent optical receiver 30 includes the optical hybrid circuit 10 of the first embodiment described above.

  The coherent optical receiver 30 also generates an LO light and outputs it to the optical hybrid circuit 10 as an LO light source 31 as a local oscillation light generator, and photoelectric conversion for converting each output optical signal of the optical hybrid circuit 10 into an electrical signal. Parts 32a and 32b. As the photoelectric conversion units 32a and 32b, specifically, a differential photo diode (BPD) is used. An in-phase component output signal is input to each of the two photodiodes of the BPD 32a, and an orthogonal component output signal is input to each of the two photodiodes of the BPD 32b.

  The coherent optical receiver 30 receives analog electric signals output from the photoelectric conversion units 32a and 32b, and inputs AD digital units 33a and 33b that output digital electric signals. And a digital arithmetic circuit 34 as a phase estimation unit for estimation.

  A monolithic integrated circuit is preferably used as the optical hybrid circuit 10 in order to reduce the size of the coherent optical receiver 30.

  Next, the operation of the coherent optical receiver 30 will be described below.

  First, QPSK signal light and LO light synchronized with the QPSK signal light are input to the input channel 11 of the optical hybrid circuit 10.

  In the optical hybrid circuit 10, these signal lights are self-imaged by multimode interference in accordance with the relative phase difference Δφ between the LO light and the QPSK signal light, and the signal light is output from each of the four output channels 12. The

  FIG. 24 is a diagram showing the transmittance in each output channel when QPSK signal light is input to the optical receiver shown in FIG. FIG. 24 shows the transmittance of each output channel when (a) Δφ = 0, (b) Δφ = π, (c) Δφ = −π / 2, and (d) Δφ = π / 2. . The transmittance ratios of the four output lights in the relative phase difference Δφ are (a) 1: 0: 2: 1, (b) 1: 2: 0: 1, (c) 0: 1: 1: 2, (D) 2: 1: 1: 0.

  The signal light from each output channel is input to the BPDs 32a and 32b.

  In the BPDs 32a and 32b, a current corresponding to +1 is output with respect to the input to the upper photodiode, and a current corresponding to -1 is output with respect to the input to the lower photodiode, both to the upper and lower parts. No current is output for the simultaneous input. As described above, the BPDs 32a and 32b convert the output signal light into an electric signal and output it to the AD conversion units 33a and 33b.

  The AD converters 33 a and 33 b that receive the analog electrical signals output from the BPDs 32 a and 32 b convert the analog electrical signals into digital electrical signals and output the digital electrical signals to the digital arithmetic circuit 34.

  The digital arithmetic circuit 34 receives a digital electrical signal, estimates the phase, and outputs the estimated phase. In this way, the coherent receiver 30 demodulates the input QPSK signal light.

  According to the coherent receiver 30 of the present embodiment described above, it has a small size and is excellent in optical performance.

  FIG. 25 is a diagram illustrating another embodiment of the optical receiver disclosed in this specification.

  The optical receiver 30a of the present embodiment inputs DQPSK signal light.

  The coherent optical receiver 30a includes a 1: 2 MMI coupler 35 that receives the DQPSK signal light, divides it into two, and outputs it. The two signal lights output from the 1: 2 MMI coupler 35 propagate through the two waveguides 36 a and 36 b and enter the optical hybrid circuit 10. Here, the optical path length of the waveguide 36a is longer than the optical path length of the waveguide 36b by an optical path length of one bit of the DQPSK signal light.

  The two DQPSK signal lights input to the optical hybrid circuit 10 are different in phase by one bit from each other. Therefore, self-imaging occurs due to multimode interference in the optical hybrid circuit 10, and signals are output from the four output channels 12. Light is output. Other operations of the coherent optical receiver 30a are the same as those in the above-described embodiment.

  In the present invention, the optical hybrid circuit of each embodiment described above and the optical receiver provided with such an optical hybrid circuit can be appropriately changed without departing from the gist of the present invention. In addition, the requirements in the above-described embodiment or modification can be appropriately interchanged between the embodiment and the modification. For example, when direct detection is considered, when BPSK signal light is input, the number of output channels is two. When 8PSK signal light is input, the number of output channels is eight.

  All examples and conditional words mentioned herein are intended for educational purposes to help the reader deepen and understand the inventions and concepts contributed by the inventor. All examples and conditional words mentioned herein are to be construed without limitation to such specifically stated examples and conditions. Also, such exemplary mechanisms in the specification are not related to showing the superiority and inferiority of the present invention. While embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions or modifications can be made without departing from the spirit and scope of the invention.

  Regarding the above-described embodiments, the following additional notes are disclosed.

(Appendix 1)
Multiple input channels,
Multiple output channels,
A multimode interference coupler in which the plurality of input channels are connected to one end and the plurality of output channels are connected to the other end;
With
The multimode interference coupler is:
A first portion whose width gradually decreases from the one end portion toward the other end portion side, and the other end from the one end portion side while being connected to the first portion and maintaining the width of the connection portion An optical waveguide device comprising: a second portion extending toward the portion side; and a third portion connected to the second portion and gradually increasing in width from the one end portion side toward the other end portion.

(Appendix 2)
The width of the first portion is defined by a pair of opposing side portions, and the shape of each of the pair of side portions is a straight line according to Supplementary Note 1.

(Appendix 3)
The width of the third portion is defined by a pair of opposing side portions, and the shape of each of the pair of side portions is a straight line according to appendix 1 or 2.

(Appendix 4)
4. The optical waveguide element according to claim 1, wherein the first portion and the third portion have the same length in a direction from the one end portion toward the other end portion.

(Appendix 5)
A multilevel phase shift keying signal light is input to any one of the plurality of input channels, and a difference in light intensity between the signal lights output from the plurality of output channels is determined by the multiphase shift keying. Any one of Supplementary notes 1 to 4, wherein a length of the second portion in a direction from the one end side to the other end side is determined so as to be within 6 dB on the basis of the light intensity of the signal light. The optical waveguide device according to item.

(Appendix 6)
The optical waveguide device according to any one of appendices 1 to 5, comprising two input channels and four output channels.

(Appendix 7)
The optical waveguide device according to any one of appendices 1 to 6, wherein the optical waveguide device is a monolithic integrated circuit.

(Appendix 8)
Multiple input channels,
Multiple output channels,
A multimode interference coupler in which the plurality of input channels are connected to one end and the plurality of output channels are connected to the other end;
With
The multimode interference coupler is:
A first portion whose width gradually decreases from the one end portion toward the other end portion side, and the other end from the one end portion side while being connected to the first portion and maintaining the width of the connection portion An optical waveguide element comprising: a second portion extending toward the portion side; and a third portion connected to the second portion and gradually increasing in width from the one end side toward the other end portion. Optical receiver.

(Appendix 9)
The optical receiver according to appendix 8, wherein the optical waveguide element is a monolithic integrated circuit.

(Appendix 10)
Supplementary note 9 or 10 comprising: a photoelectric conversion unit that converts each output optical signal of the optical waveguide element into an electric signal; and a phase estimation unit that inputs each electric signal output from the photoelectric conversion unit and estimates a phase. The optical receiver described.

(Appendix 11)
A multimode interference coupler that propagates light from one end to the other end,
A first portion whose width gradually decreases from the one end portion toward the other end portion side, and the other end from the one end portion side while being connected to the first portion and maintaining the width of the connection portion A multi-mode interference coupler, comprising: a second portion extending toward the portion side; and a third portion connected to the second portion and gradually increasing in width from the one end side toward the other end portion.

10 Optical hybrid circuit (optical waveguide device)
DESCRIPTION OF SYMBOLS 11 Input channel 12 Output channel 13 Multimode interference coupler 13a 1st part of a multimode interference coupler 13b 2nd part of a multimode interference coupler 13c 3rd part of a multimode interference coupler 13e A pair of side parts 13f 3rd 3rd A pair of side portions 14 One end portion 15 The other end portion 30 Optical receiver 31 Local oscillation light generation portion 32a, 32b Differential photodiode (photoelectric conversion portion)
33a, 33b AD conversion unit 34 Digital arithmetic circuit (phase estimation unit)
35 The length of the width LM1 first portion width W _M second part of the multimode interference coupler of the one end portion of the upper cladding layer 44 the mesa W _S multimode interference coupler delay unit 40 the substrate 41 lower cladding layer 42 a core layer 43 LM2 Length of first part and second part combined LM3 Length of multimode interference coupler CL Center axis in width direction

Claims (5)

An optical waveguide device used as an optical hybrid circuit of a coherent optical receiver ,
Multiple input channels,
Multiple output channels,
A multimode interference coupler in which the plurality of input channels are connected to one end and the plurality of output channels are connected to the other end;
With
The multimode interference coupler is:
A first portion whose width gradually decreases from the one end portion toward the other end portion side, and the other end from the one end portion side while being connected to the first portion and maintaining the width of the connection portion A second part extending toward the part side, and a third part connected to the second part and gradually increasing in width from the one end side toward the other end part,
An optical fiber in which the ratio of the length of the second portion and the length of the multimode interference coupler in the direction from the one end side to the other end side is in the range of 100/449 to 110 / 446.3. Waveguide element.   2. The optical waveguide device according to claim 1, wherein a width of the first portion is defined by a pair of opposing side portions, and a shape of each of the pair of side portions is a straight line.   3. The optical waveguide device according to claim 1, wherein a width of the third portion is defined by a pair of opposing side portions, and a shape of each of the pair of side portions is a straight line.   4. The optical waveguide element according to claim 1, wherein the first portion and the third portion have the same length in a direction from the one end portion toward the other end portion. 5. An optical waveguide device used as an optical hybrid circuit,
Multiple input channels,
Multiple output channels,
A multimode interference coupler in which the plurality of input channels are connected to one end and the plurality of output channels are connected to the other end;
With
The multimode interference coupler is:
A first portion whose width gradually decreases from the one end portion toward the other end portion side, and the other end from the one end portion side while being connected to the first portion and maintaining the width of the connection portion A second part extending toward the part side, and a third part connected to the second part and gradually increasing in width from the one end part side toward the other end part. the ratio of the length of said second portion and said multi-mode interference coupler from part side in the direction toward the end portion side of the other is an optical waveguide element is in the range of 100 / 449-110 / 446.3, the Coherent optical receiver equipped.

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